The present invention relates to a brushless motor of the type wherein permanent magnets are mounted on a rotor and windings are wound about the salient cores of a stator.
Generally, brushless motors of this type generate rotary magnetic fields by changing the phase of current passing through windings about a stator having salient poles for m phases.
The width of an induced voltage waveform equivalent to each torque constant should preferably be broad enough to make the torque imparted to the rotor large and to make the minimum torque ripples small.
In providing the windings on the stator core having the salient poles, a method of providing only windings belonging to the same phase on a single salient pole is effective to keep the coil ends from becoming superposed one upon another.
With such windings, however, the induced voltage waveform becomes largely dependent upon a relation between the width of the salient pole and that of the magnetic pole.
In a prior method as shown in FIG. 4, if the widths of the salient poles 3a, 3b, 3c, 3d, and so on, are made smaller than the widths of the magnetic poles 2a, 2b, 2c, and so on, then the waveform of an induced voltage V becomes like that shown in FIG. 5. The waveform has a large portion near to zero in the vicinity of the zero cross point and has a narrower width. Therefore, it brings about a problem that the effective value of torque becomes smaller.
However, if the widths of the salient poles 3a, 3b, and so on, are larger than those of the magnetic poles 2a, 2b, and so on, then the waveform of an induced voltage V also becomes like that shown in FIG. 5, resulting in the same problem as above.
Therefore, it is desirable to make the widths of the salient poles 3a, 3b, and so on, equal to the widths of the magnetic poles 2a, 2b, and so on.
Furthermore, on, condition that the widths of the salient poles 3a, 3b, and so on, are made almost equal to the widths of the magnetic poles 2a, 2b, and so on, the spaces between the salient poles 3a, 3b, and so on, about which the stator windings belonging to the same phase are also wound, are made equal to each other at an integral multiple of the space between the magnetic poles 2a, 2b, and so on. In this case, the adjoining salient poles 3a, 3b, and so on, must be arranged so as to have an angle corresponding to the electrical angle for the m phases. Therefore, there arises a problem the opening between the opposite tips of the adjoining salient poles becomes broad and the effective air gap becomes larger.
In particular, in a two-phase motor as shown in FIG. 6, for example, if the number of magnetic poles is set at P, the adjoining salient poles must have an electrical angle of an odd multiple of 90.degree.. Thus, in order to make the widths of the magnetic poles 2a, 2b, and so on, equal to the widths of the salient poles 3a, 3b, and so on, at least an electrical angle of 270.degree. [mechanical angle of 270.degree./(P/2)] must be ensured.
However, in this case, since the widths of the salient poles 3a, 3b, and so on, are equal to the widths of the magnetic poles 2a, 2b, and so on, the angle becomes one corresponding to an electrical angle of 180.degree.. Therefore, the areas where the magnets 2a, 2b, and so on, face the cores 3a, 3b, and so on, become as small as 180.degree./270.degree.=2/3 to make the effective air gap larger and, thereby, degrade the characteristics of the motor.